Because of the effects of quantum-confinement, the emission color of semiconductor nanocrystals (NCs) can be modified dramatically by simply changing NC size. Spectral tunability, large photoluminescence (PL) quantum yields, and high photostability make NCs attractive for applications in such light-emitting technologies as displays, fluorescence tagging, solid-state lighting, and lasing. An important concern associated with light-emitting applications of NCs is the difficulty in achieving electrical pumping. Thus far, all attempts to directly contact NCs electrically have met with limited success, largely due to the presence of an insulating organic capping layer. Here the indirect exciton injection into NCs is explored via a non-contact, non-radiative energy transfer (ET) from a proximal quantum well (QW) that can be pumped either electrically or optically. Our theoretical treatment and direct experimental measurements indicate that this transfer is fast enough to compete with exciton recombination in the QW and results in greater than 50% QW-to-NC ET efficiencies in the tested devices. Furthermore, the measured ET rates are sufficiently large to provide NC pumping not only in the spontaneous but also in the stimulated emission regime, indicating the feasibility of ET-pumped, NC-based optical amplifiers and lasers.
Several programs worldwide emphasize the need for efficient solid-state emitters in applications ranging from displays and traffic signs to solid-state lighting. Semiconductor nanocrystals (NCs) have been considered promising nanoscale color-selectable emitters that combine high photoluminescence (PL) quantum yields with chemical flexibility and processibility. Such quantum yields (QY) may potentially be as high as 100%. Even in the form of a single monolayer NCs can produce significant power outputs on the order of Watts per cm2 (estimated value for an NC packing density of 1012 cm−2, a radiative lifetime of 20 ns, and a moderate QY of 20%).
One approach to NC-based, electrically pumped light emitting devices utilizes hybrid organic/inorganic structures, in which the charges are delivered to NCs via the organic network and/or percolated NC subsystem. The performance of these devices is, however, limited by low carrier mobilities in both the organic and NC components and by the poor stability of organic molecules with respect to photooxidation.
Despite the gradual progress, problems have remained. After long and careful research, a new approach has now been developed for the pumping of light emitters via non-radiative energy transfer.